1. Field of the Invention
The present invention relates to the parietal cooling of the walls of combustion chambers of turbomachines, and more precisely to a method of manufacturing a device for creating an inner parietal cooling film for a combustion chamber wall. For the sake of simplicity, the device for creating the parietal film (i.e. the annular member having an annular chamber provided with a row of air inlet apertures and an annular outlet slit through which air enters the combustion chamber) is hereinafter referred to as "the cooling film device".
2. Summary of the Prior Art
The internal temperature of turbomachine combustion chambers is very high and it is current practice to use the principle known as "film cooling" to cool the walls of the chamber. This involves projecting a film of air over the internal face of the wall so as to protect it from the direct action of the flame.
The main problem posed by this process is that of slowing down sufficiently to a desired speed, equal to that of the gases in the combustion chamber, the internal cooling air so that it flows along the inner face of the wall to be cooled and forms an even film thereon. The Applicant has already proposed a solution to this problem in U.S. Pat. No. 4,329,848 wherein an annular protuberance is formed in which a radial portion of its wall is perforated with several rows of air inlet openings in a staggered arrangement, the air flowing along the downstream wall of the chamber through an annular slit provided between said wall and an inner downstream tongue.
The present invention proposes a method of constructing this type of cooling film device which is easy to carry out.
In the combustion chambers of modern high output turbomachines, the dimensions of the chambers have been able to be decreased, but this reduction of dimensions, advantageous to the weight/power performance ratios of the turbomachine, increases the difficulties of construction of said chambers.
Indeed, the annular members from which the combustion chambers are made must form geometrical structures substantially identical with those of the members of larger volume chambers, but in considerably reduced dimensions. Consequently, the conventional methods of construction are very difficult to carry out.
Among these conventional methods mention may be made of U.S. Pat. No. 4,109,459 in which the cooling film device is formed in two parts, the upstream wall and the tongue being constituted by the downstream edge of an inner upstream annular member, whereas the body of the annular pocket and the downstream wall are constituted by a downstream annular member surrounding the inner upstream member on which it is welded. U.S. Pat. No. 4,485,630 discloses a simplification of this method of construction, still with a cooling film device in two parts.
FR Pat. No. 2,440,524 also shows cooling film devices, but of which the construction does not present any particular difficulties as the downstream shells of the combustion chamber have a diameter decidedly greater than that of the upstream shells which they surround, and the air inlet pocket of the cooling film device has no inlet protuberance, nor a downstream annular slit which is narrowed relative to the pocket.
One object of the present invention is to provide, in simple manner, an annular pocket for the admission of air and the settling down of the cooling film, of a diameter more substantial than that of the downstream annular slit for the ejection of the air into the combustion chamber, and in an annular member producing a monobloc cooling film device as opposed to a two-part device as in the above-mentioned patents, this with a view to increasing appreciably the structural rigidity of the device while simplifying the structure of the walls of the combustion chambers.
A further object of the invention is to permit the. production of cooling film devices of very small dimensions which retain the properties of large volume devices, of which the methods of construction are not readily adaptable to present smaller devices.